Temperature Sensors

["Fred L. DeRoos" <flderoos@mmmpcc.org>]

> Date: Wed, 28 May 1997 17:40:12 -0400 (EDT)
> From: "Stephen J. Siblock" <steve.siblock@sheridanc.on.ca>
> Subject: temperature sensors (on HVAC)
> 
> Is there a way to correct the temperature reading on the HVAC system?
> Mine is about 7 degrees C "too cold". 

Steve,

I'm not sure which car you have, but the 200s use a pair of negative
temperature coefficient thermistors to sense the outside temperature. 
One of them is located in the plenum below the window (under the hood on
the passenger side) and the other is located in front of the AC
condensor approximately in the middle of the car.  Both thermistors are
identical and are 1000 ohms at 25 deg C.  They have an R value of 9.10
and can be purchased from Digikey Corp. in Thief River Falls, MN for
approximately $2 each.  The part number is KC016N-ND.  The reason you
are reading low, is that the AC head unit reads both sensors, then
displays the one that senses the lower of the two temperatures.  This is
done to prevent the display of an artificially high temperature due to
engine or sun heating.  The temp may still be a little high, but at
least it would be the lower of the two.  The problem is when one of
these sensors goes bad, it typically increases in resistance.  That
means that your temp display will show low.  

The display will operate with only one sensor, so you can do one of two
things.  Best choice is to check the resistance of each sensor at
approximately 25 C and see which one is reading high, then replace it. 
Second approach is to unplug each sensor separately and see which one
makes the temp read correctly (assuming you haven't been parked in the
bright sun or just run the car and the engine is hot.  The front one by
the condensor was bad on my 1990 200 TQ.  When I unplugged it, the temp
read ok, but often needed a few minutes of driving to blow air over it
and reach a stable temp indication.

To replace the thermistor, you will need to remove the bad assembly,
then remove the plastic tip from the holder.  The original thermistor is
soldered to the two wires and fits into a plastic tube.  The new one is
just slightly larger, so you will need to insert a screwdriver into the
plastic tube and enlarge it approximately 0.02 0.04 inches.  Audi would
be happy to sell you a replacement, however, they list for approximately
$100.  

EMAIL me if you have any questions.  It is an easy repair.

Fred